Mobile telecommunication users travel between multiple Public Land Mobile Networks (PLMNs) utilizing the same telephone number and the same mobile station to originate outgoing calls and to receive incoming calls. The ability for a cellular subscriber to automatically make and receive voice calls, to send and receive data or access other services when traveling outside the geographical coverage area of the home PLMN (H-PLMN) by means of using a Visited PLMN (V-PLMN) can be defined as “roaming”. The differentiation between a H-PLMN and a V-PLMN is based on the availability of the subscriber data in the home subscriber register of the network, which may be a Home Location Register (HLR) in a GSM technology-based PLMN. The differentiation between H-PLMN and V-PLMN is therefore always related to a specific subscriber. If this subscriber roams into a V-PLMN then the home subscriber register of this V-PLMN does not comprise subscriber data or a subscriber record related to this specific subscriber. To establish a call to a subscriber served by a call control node in a V-PLMN it is necessary to route the call to the call control node of the V-PLMN.
If a calling subscriber and a called subscriber are both served by the same network which is a V-PLMN for the called subscriber, two inter-network call legs between this V-PLMN and the H-PLMN of the called subscriber are needed to route the call to the call control node serving the called subscriber.
The conventional way of routing a call between two subscribers that are located within the same network and wherein at least the called subscriber has a subscription to a home network which is different to the current serving network requires to route the call through the home network of the called subscriber. A gateway node in the V-PLMN may route the call towards a gateway node in the H-PLMN of the called subscriber. The gateway node in the H-PLMN may be a Gateway Mobile Switching Center (GMSC). This gateway node interrogates the home subscriber register, which may be a HLR that keeps the subscriber record of the called subscriber to get information about the current serving call control node of the called subscriber. After getting this information the call is routed back into the V-PLMN of the subscriber. This effect—which is commonly referred to as “tromboning”—is wasteful on network resources, degrades user experience due to this long-lasting call setup and is expensive for network operators and end users.
Standard 3GPP TS 23.119 Version 4.0.0 from September 2001 introduced a Gateway Location Register (GLR) which is located in the visited network. The GLR is a node between the call control node in the visited network and the home subscriber register in the home network. FIG. 1 depicts a set-up according to the prior art which implements a GLR 121 according to the standard mentioned above. Two subscribers 101 and 102 are served via radio access nodes 128 and 127 by call control nodes 124, 123 in a network “A” 120. The call control nodes 123, 124 are depicted as MSC-Servers (MSC-S). It may also be possible that the call control nodes are Serving “General Packet Radio Service” (GPRS) Support nodes (SGSNs) in an UMTS-based network. Network A 120 is a V-PLMN for subscriber B 101 which is the called subscriber in this set-up. FIG. 1 further depicts two Media Gateways (M-MGW) 125, 126 which are adapted to route the payload data between the subscribers 102, 101. Payload data may be voice or other data. The M-MGWs 125, 126 are controlled by the call control nodes 123, 124. It may be possible that there are more than two call control nodes 123, 124 and/or more than two media gateways 125, 126 through which control data and payload data is routed. Both subscribers 101, 102 are represented by User Equipment (UEs) which comprise subscriptions to network operators in their H-PLMNs. This can be performed by introducing a SIM-card into the UEs 101, 102 which comprises subscription information. The network A 120 further comprises an international gateway (IGW) 122 which is adapted to connect the network A 120 to another network B 110. It is therefore possible to send connection data between both networks A 120 and B 110 to route calls between networks. The IGW 122 is connected to a gateway node GMSC 112 in the network B 110 which is the H-PLMN of subscriber B 101. The GMSC 112 can also act as an IGW for network B 110. The GMSC 112 is connected to a home location register HLR 111 which comprises a subscriber record of subscriber B. The HLR 111 in network B 110 and the GLR 121 in network A 120 are connected such that the GLR 121 is able to fetch subscriber data from the HLR 111 when the subscriber B 101 performs the first location update procedure in the visited network.
FIG. 2 shows a call establishment between a UE A 102 which is the UE of subscriber A and a UE B 101 which is the UE of subscriber B. UE A 102 sets up a call towards UE B 102 by informing the serving call control node MSC/VLR A 123. The MSC/VLR A 123 is aware of the fact that the called subscriber UE B 101 does not have a subscription to the network A 120 based on the called party number and will forward the call set-up message 201 to the international gateway IGW 122. The IGW 122 routes the call set-up message, comprising the MSISDN (Mobile Subscriber ISDN), to the GMSC 112 of network B 110. GMSC 112 fetches the subscriber data via an “Inv: Send Routing Information” message 202 from the HLR 111. This message comprises the MSISDN of UE A 102. The HLR 111 requests a Roaming Number from the VLR (Visited Location Register) of the MSC/VLR B 124 via the GLR 121 by a message “Inv: Provide Roaming Number” 203, comprising the address of the VLR (Visited Location Register) serving the called subscriber UE B 101 and the IMSI (International Mobile Subscriber Identity) of the UE A 102. The GLR 121 forwards this request to the MSC/VLR B 124 with a message “Inv: Provide Roaming Number” 204. As an answer to this request the Roaming Number and the IMSI is sent from the MSC/VLR B 124 to the GLR 121 in a message “Res: Provide Roaming Number” 205. The GLR 121 forwards this response in a further message towards the HLR 111. The HLR 111 sends the routing information which comprises the roaming number to the GMSC 112 which routes the call via the IGW 122 to the MSC/VLR B 124 which establishes a connection via the radio access with UE B 101. A dashed line shows the border between the H-PLMN and the V-PLMN.
The Roaming Number must always be fetched from the HLR 111 via the GMSC 112 to establish a call between two subscribers in a V-PLMN 120 of the called subscriber. The GLR 121 is only used to optimize the location updating and the handling of subscriber profile data across network boundaries. When a subscriber is roaming within a V-PLMN 120 the GLR 121 acts during the location update procedure like a home subscriber register towards the call control node in the V-PLMN. This solution reduces the location update related signaling between the V-PLMN and the H-PLMN when the subscriber registers at a new location within the V-PLMN using the “Location Update” procedure. The GLR 121 does not reduce the signaling between visited network 120 and home network 110 of a called subscriber 101 when the calling subscriber 102 is located in the same network as the called subscriber 101.